Network Working Group C. Jennings, Ed.
Request for Comments: 5626 Cisco Systems
Updates: 3261, 3327 R. Mahy, Ed.
Category: Standards Track Unaffiliated
F. Audet, Ed.
Skype Labs
October 2009
Managing Client-Initiated Connectionsin the Session Initiation Protocol (SIP)
Abstract
The Session Initiation Protocol (SIP) allows proxy servers to
initiate TCP connections or to send asynchronous UDP datagrams to
User Agents in order to deliver requests. However, in a large number
of real deployments, many practical considerations, such as the
existence of firewalls and Network Address Translators (NATs) or the
use of TLS with server-provided certificates, prevent servers from
connecting to User Agents in this way. This specification defines
behaviors for User Agents, registrars, and proxy servers that allow
requests to be delivered on existing connections established by the
User Agent. It also defines keep-alive behaviors needed to keep NAT
bindings open and specifies the usage of multiple connections from
the User Agent to its registrar.
Status of This Memo
This document specifies an Internet standards track protocol for the
Internet community, and requests discussion and suggestions for
improvements. Please refer to the current edition of the "Internet
Official Protocol Standards" (STD 1) for the standardization state
and status of this protocol. Distribution of this memo is unlimited.
Copyright Notice
Copyright (c) 2009 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents
(http://trustee.ietf.org/license-info) in effect on the date of
publication of this document. Please review these documents
carefully, as they describe your rights and restrictions with respect
to this document. Code Components extracted from this document must
include Simplified BSD License text as described in Section 4.e of
Jennings, et al. Standards Track [Page 1]

RFC 5626 Client-Initiated Connections in SIP October 20091. Introduction
There are many environments for SIP [RFC3261] deployments in which
the User Agent (UA) can form a connection to a registrar or proxy but
in which connections in the reverse direction to the UA are not
possible. This can happen for several reasons, but the most likely
is a NAT or a firewall in between the SIP UA and the proxy. Many
such devices will only allow outgoing connections. This
specification allows a SIP User Agent behind such a firewall or NAT
to receive inbound traffic associated with registrations or dialogs
that it initiates.
Most IP phones and personal computers get their network
configurations dynamically via a protocol such as the Dynamic Host
Configuration Protocol (DHCP) [RFC2131]. These systems typically do
not have a useful name in the Domain Name System (DNS) [RFC1035], and
they almost never have a long-term, stable DNS name that is
appropriate for use in the subjectAltName of a certificate, as
required by [RFC3261]. However, these systems can still act as a
Transport Layer Security (TLS) [RFC5246] client and form outbound
connections to a proxy or registrar that authenticates with a server
certificate. The server can authenticate the UA using a shared
secret in a digest challenge (as defined in Section 22 of RFC 3261)
over that TLS connection. This specification allows a SIP User Agent
who has to initiate the TLS connection to receive inbound traffic
associated with registrations or dialogs that it initiates.
The key idea of this specification is that when a UA sends a REGISTER
request or a dialog-forming request, the proxy can later use this
same network "flow" -- whether this is a bidirectional stream of UDP
datagrams, a TCP connection, or an analogous concept in another
transport protocol -- to forward any incoming requests that need to
go to this UA in the context of the registration or dialog.
For a UA to receive incoming requests, the UA has to connect to a
server. Since the server can't connect to the UA, the UA has to make
sure that a flow is always active. This requires the UA to detect
when a flow fails. Since such detection takes time and leaves a
window of opportunity for missed incoming requests, this mechanism
allows the UA to register over multiple flows at the same time. This
specification also defines two keep-alive schemes. The keep-alive
mechanism is used to keep NAT bindings fresh, and to allow the UA to
detect when a flow has failed.
Jennings, et al. Standards Track [Page 4]

RFC 5626 Client-Initiated Connections in SIP October 20092. Conventions and Terminology
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in [RFC2119].
2.1. Definitions
Authoritative Proxy: A proxy that handles non-REGISTER requests for
a specific Address-of-Record (AOR), performs the logical Location
Server lookup described in [RFC3261], and forwards those requests
to specific Contact URIs. (In [RFC3261], the role that is
authoritative for REGISTER requests for a specific AOR is a
Registration Server.)
Edge Proxy: An edge proxy is any proxy that is located topologically
between the registering User Agent and the Authoritative Proxy.
The "first" edge proxy refers to the first edge proxy encountered
when a UA sends a request.
Flow: A Flow is a transport-layer association between two hosts that
is represented by the network address and port number of both ends
and by the transport protocol. For TCP, a flow is equivalent to a
TCP connection. For UDP a flow is a bidirectional stream of
datagrams between a single pair of IP addresses and ports of both
peers. With TCP, a flow often has a one-to-one correspondence
with a single file descriptor in the operating system.
Flow Token: An identifier that uniquely identifies a flow which can
be included in a SIP URI (Uniform Resource Identifier [RFC3986]).
reg-id: This refers to the value of a new header field parameter
value for the Contact header field. When a UA registers multiple
times, each for a different flow, each concurrent registration
gets a unique reg-id value.
instance-id: This specification uses the word instance-id to refer
to the value of the "sip.instance" media feature tag which appears
as a "+sip.instance" Contact header field parameter. This is a
Uniform Resource Name (URN) that uniquely identifies this specific
UA instance.
"ob" Parameter: The "ob" parameter is a SIP URI parameter that has a
different meaning depending on context. In a Path header field
value, it is used by the first edge proxy to indicate that a flow
token was added to the URI. In a Contact or Route header field
value, it indicates that the UA would like other requests in the
same dialog to be routed over the same flow.
Jennings, et al. Standards Track [Page 5]

RFC 5626 Client-Initiated Connections in SIP October 2009
outbound-proxy-set: A set of SIP URIs (Uniform Resource Identifiers)
that represents each of the outbound proxies (often edge proxies)
with which the UA will attempt to maintain a direct flow. The
first URI in the set is often referred to as the primary outbound
proxy and the second as the secondary outbound proxy. There is no
difference between any of the URIs in this set, nor does the
primary/secondary terminology imply that one is preferred over the
other.
3. Overview
The mechanisms defined in this document are useful in several
scenarios discussed below, including the simple co-located registrar
and proxy, a User Agent desiring multiple connections to a resource
(for redundancy, for example), and a system that uses edge proxies.
This entire section is non-normative.
3.1. Summary of Mechanism
Each UA has a unique instance-id that stays the same for this UA even
if the UA reboots or is power cycled. Each UA can register multiple
times over different flows for the same SIP Address of Record (AOR)
to achieve high reliability. Each registration includes the
instance-id for the UA and a reg-id label that is different for each
flow. The registrar can use the instance-id to recognize that two
different registrations both correspond to the same UA. The
registrar can use the reg-id label to recognize whether a UA is
creating a new flow or refreshing or replacing an old one, possibly
after a reboot or a network failure.
When a proxy goes to route a message to a UA for which it has a
binding, it can use any one of the flows on which a successful
registration has been completed. A failure to deliver a request on a
particular flow can be tried again on an alternate flow. Proxies can
determine which flows go to the same UA by comparing the instance-id.
Proxies can tell that a flow replaces a previously abandoned flow by
looking at the reg-id.
When sending a dialog-forming request, a UA can also ask its first
edge proxy to route subsequent requests in that dialog over the same
flow. This is necessary whether the UA has registered or not.
UAs use a simple periodic message as a keep-alive mechanism to keep
their flow to the proxy or registrar alive. For connection-oriented
transports such as TCP this is based on carriage-return and line-feed
Jennings, et al. Standards Track [Page 6]

RFC 5626 Client-Initiated Connections in SIP October 2009
sequences (CRLF), while for transports that are not connection
oriented, this is accomplished by using a SIP-specific usage profile
of STUN (Session Traversal Utilities for NAT) [RFC5389].
3.2. Single Registrar and UA
In the topology shown below, a single server is acting as both a
registrar and proxy.
+-----------+
| Registrar |
| Proxy |
+-----+-----+
|
|
+----+--+
| User |
| Agent |
+-------+
User Agents that form only a single flow continue to register
normally but include the instance-id as described in Section 4.1.
The UA also includes a "reg-id" Contact header field parameter that
is used to allow the registrar to detect and avoid keeping invalid
contacts when a UA reboots or reconnects after its old connection has
failed for some reason.
For clarity, here is an example. Bob's UA creates a new TCP flow to
the registrar and sends the following REGISTER request.
REGISTER sip:example.com SIP/2.0
Via: SIP/2.0/TCP 192.0.2.2;branch=z9hG4bK-bad0ce-11-1036
Max-Forwards: 70
From: Bob <sip:bob@example.com>;tag=d879h76
To: Bob <sip:bob@example.com>
Call-ID: 8921348ju72je840.204
CSeq: 1 REGISTER
Supported: path, outbound
Contact: <sip:line1@192.0.2.2;transport=tcp>; reg-id=1;
;+sip.instance="<urn:uuid:00000000-0000-1000-8000-000A95A0E128>"
Content-Length: 0
The registrar challenges this registration to authenticate Bob. When
the registrar adds an entry for this contact under the AOR for Bob,
the registrar also keeps track of the connection over which it
received this registration.
Jennings, et al. Standards Track [Page 7]

RFC 5626 Client-Initiated Connections in SIP October 2009
The registrar saves the instance-id
("urn:uuid:00000000-0000-1000-8000-000A95A0E128") and reg-id ("1")
along with the rest of the Contact header field. If the instance-id
and reg-id are the same as a previous registration for the same AOR,
the registrar replaces the old Contact URI and flow information.
This allows a UA that has rebooted to replace its previous
registration for each flow with minimal impact on overall system
load.
When Alice sends a request to Bob, his authoritative proxy selects
the target set. The proxy forwards the request to elements in the
target set based on the proxy's policy. The proxy looks at the
target set and uses the instance-id to understand if two targets both
end up routing to the same UA. When the proxy goes to forward a
request to a given target, it looks and finds the flows over which it
received the registration. The proxy then forwards the request over
an existing flow, instead of resolving the Contact URI using the
procedures in [RFC3263] and trying to form a new flow to that
contact.
As described in the next section, if the proxy has multiple flows
that all go to this UA, the proxy can choose any one of the
registration bindings for this AOR that has the same instance-id as
the selected UA.
3.3. Multiple Connections from a User Agent
There are various ways to deploy SIP to build a reliable and scalable
system. This section discusses one such design that is possible with
the mechanisms in this specification. Other designs are also
possible.
In the example system below, the logical outbound proxy/registrar for
the domain is running on two hosts that share the appropriate state
and can both provide registrar and outbound proxy functionality for
the domain. The UA will form connections to two of the physical
hosts that can perform the authoritative proxy/registrar function for
the domain. Reliability is achieved by having the UA form two TCP
connections to the domain.
Jennings, et al. Standards Track [Page 8]

RFC 5626 Client-Initiated Connections in SIP October 2009
+-------------------+
| Domain |
| Logical Proxy/Reg |
| |
|+-----+ +-----+|
||Host1| |Host2||
|+-----+ +-----+|
+---\------------/--+
\ /
\ /
\ /
\ /
+------+
| User |
| Agent|
+------+
The UA is configured with multiple outbound proxy registration URIs.
These URIs are configured into the UA through whatever the normal
mechanism is to configure the proxy address and AOR in the UA. If
the AOR is alice@example.com, the outbound-proxy-set might look
something like "sip:primary.example.com" and "sip:
secondary.example.com". Note that each URI in the outbound-proxy-set
could resolve to several different physical hosts. The
administrative domain that created these URIs should ensure that the
two URIs resolve to separate hosts. These URIs are handled according
to normal SIP processing rules, so mechanisms like DNS SRV [RFC2782]
can be used to do load-balancing across a proxy farm. The approach
in this document does not prevent future extensions, such as the SIP
UA configuration framework [CONFIG-FMWK], from adding other ways for
a User Agent to discover its outbound-proxy-set.
The domain also needs to ensure that a request for the UA sent to
Host1 or Host2 is then sent across the appropriate flow to the UA.
The domain might choose to use the Path header approach (as described
in the next section) to store this internal routing information on
Host1 or Host2.
When a single server fails, all the UAs that have a flow through it
will detect a flow failure and try to reconnect. This can cause
large loads on the server. When large numbers of hosts reconnect
nearly simultaneously, this is referred to as the avalanche restart
problem, and is further discussed in Section 4.5. The multiple flows
to many servers help reduce the load caused by the avalanche restart.
If a UA has multiple flows, and one of the servers fails, the UA
delays a recommended amount of time before trying to form a new
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RFC 5626 Client-Initiated Connections in SIP October 2009
connection to replace the flow to the server that failed. By
spreading out the time used for all the UAs to reconnect to a server,
the load on the server farm is reduced.
Scalability is achieved by using DNS SRV [RFC2782] to load-balance
the primary connection across a set of machines that can service the
primary connection, and also using DNS SRV to load-balance across a
separate set of machines that can service the secondary connection.
The deployment here requires that DNS is configured with one entry
that resolves to all the primary hosts and another entry that
resolves to all the secondary hosts. While this introduces
additional DNS configuration, the approach works and requires no
additional SIP extensions to [RFC3263].
Another motivation for maintaining multiple flows between the UA and
its registrar is related to multihomed UAs. Such UAs can benefit
from multiple connections from different interfaces to protect
against the failure of an individual access link.
3.4. Edge Proxies
Some SIP deployments use edge proxies such that the UA sends the
REGISTER to an edge proxy that then forwards the REGISTER to the
registrar. There could be a NAT or firewall between the UA and the
edge proxy.
+---------+
|Registrar|
|Proxy |
+---------+
/ \
/ \
/ \
+-----+ +-----+
|Edge1| |Edge2|
+-----+ +-----+
\ /
\ /
----------------------------NAT/FW
\ /
\ /
+------+
|User |
|Agent |
+------+
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RFC 5626 Client-Initiated Connections in SIP October 2009
The edge proxy includes a Path header [RFC3327] so that when the
proxy/registrar later forwards a request to this UA, the request is
routed through the edge proxy.
These systems can use effectively the same mechanism as described in
the previous sections but need to use the Path header. When the edge
proxy receives a registration, it needs to create an identifier value
that is unique to this flow (and not a subsequent flow with the same
addresses) and put this identifier in the Path header URI. This
identifier has two purposes. First, it allows the edge proxy to map
future requests back to the correct flow. Second, because the
identifier will only be returned if the user authenticates with the
registrar successfully, it allows the edge proxy to indirectly check
the user's authentication information via the registrar. The
identifier is placed in the user portion of a loose route in the Path
header. If the registration succeeds, the edge proxy needs to map
future requests (that are routed to the identifier value from the
Path header) to the associated flow.
The term edge proxy is often used to refer to deployments where the
edge proxy is in the same administrative domain as the registrar.
However, in this specification we use the term to refer to any proxy
between the UA and the registrar. For example, the edge proxy may be
inside an enterprise that requires its use, and the registrar could
be from a service provider with no relationship to the enterprise.
Regardless of whether they are in the same administrative domain,
this specification requires that registrars and edge proxies support
the Path header mechanism in [RFC3327].
3.5. Keep-Alive Technique
This document describes two keep-alive mechanisms: a CRLF keep-alive
and a STUN keep-alive. Each of these mechanisms uses a client-to-
server "ping" keep-alive and a corresponding server-to-client "pong"
message. This ping-pong sequence allows the client, and optionally
the server, to tell if its flow is still active and useful for SIP
traffic. The server responds to pings by sending pongs. If the
client does not receive a pong in response to its ping (allowing for
retransmission for STUN as described in Section 4.4.2), it declares
the flow dead and opens a new flow in its place.
This document also suggests timer values for these client keep-alive
mechanisms. These timer values were chosen to keep most NAT and
firewall bindings open, to detect unresponsive servers within 2
minutes, and to mitigate against the avalanche restart problem.
However, the client may choose different timer values to suit its
needs, for example to optimize battery life. In some environments,
Jennings, et al. Standards Track [Page 11]

RFC 5626 Client-Initiated Connections in SIP October 2009
the server can also keep track of the time since a ping was received
over a flow to guess the likelihood that the flow is still useful for
delivering SIP messages.
When the UA detects that a flow has failed or that the flow
definition has changed, the UA needs to re-register and will use the
back-off mechanism described in Section 4.5 to provide congestion
relief when a large number of agents simultaneously reboot.
A keep-alive mechanism needs to keep NAT bindings refreshed; for
connections, it also needs to detect failure of a connection; and for
connectionless transports, it needs to detect flow failures including
changes to the NAT public mapping. For connection-oriented
transports such as TCP [RFC0793] and SCTP [RFC4960], this
specification describes a keep-alive approach based on sending CRLFs.
For connectionless transport, such as UDP [RFC0768], this
specification describes using STUN [RFC5389] over the same flow as
the SIP traffic to perform the keep-alive.
UAs and Proxies are also free to use native transport keep-alives;
however, the application may not be able to set these timers on a
per-connection basis, and the server certainly cannot make any
assumption about what values are used. Use of native transport
keep-alives is outside the scope of this document.
3.5.1. CRLF Keep-Alive Technique
This approach can only be used with connection-oriented transports
such as TCP or SCTP. The client periodically sends a double-CRLF
(the "ping") then waits to receive a single CRLF (the "pong"). If
the client does not receive a "pong" within an appropriate amount of
time, it considers the flow failed.
Note: Sending a CRLF over a connection-oriented transport is
backwards compatible (because of requirements in Section 7.5 of
[RFC3261]), but only implementations which support this
specification will respond to a "ping" with a "pong".
3.5.2. STUN Keep-Alive Technique
This approach can only be used for connection-less transports, such
as UDP.
For connection-less transports, a flow definition could change
because a NAT device in the network path reboots and the resulting
public IP address or port mapping for the UA changes. To detect
this, STUN requests are sent over the same flow that is being used
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RFC 5626 Client-Initiated Connections in SIP October 2009
for the SIP traffic. The proxy or registrar acts as a limited
Session Traversal Utilities for NAT (STUN) [RFC5389] server on the
SIP signaling port.
Note: The STUN mechanism is very robust and allows the detection
of a changed IP address and port. Many other options were
considered, but the SIP Working Group selected the STUN-based
approach. Approaches using SIP requests were abandoned because
many believed that good performance and full backwards
compatibility using this method were mutually exclusive.
4. User Agent Procedures4.1. Instance ID Creation
Each UA MUST have an Instance Identifier Uniform Resource Name (URN)
[RFC2141] that uniquely identifies the device. Usage of a URN
provides a persistent and unique name for the UA instance. It also
provides an easy way to guarantee uniqueness within the AOR. This
URN MUST be persistent across power cycles of the device. The
instance ID MUST NOT change as the device moves from one network to
another.
A UA SHOULD create a Universally Unique Identifier (UUID) URN
[RFC4122] as its instance-id. The UUID URN allows for non-
centralized computation of a URN based on time, unique names (such as
a MAC address), or a random number generator.
Note: A device like a "soft phone", when first installed, can
generate a UUID [RFC4122] and then save this in persistent storage
for all future use. For a device such as a "hard phone", which
will only ever have a single SIP UA present, the UUID can include
the MAC address and be generated at any time because it is
guaranteed that no other UUID is being generated at the same time
on that physical device. This means the value of the time
component of the UUID can be arbitrarily selected to be any time
less than the time when the device was manufactured. A time of 0
(as shown in the example in Section 3.2) is perfectly legal as
long as the device knows no other UUIDs were generated at this
time on this device.
If a URN scheme other than UUID is used, the UA MUST only use URNs
for which an RFC (from the IETF stream) defines how the specific URN
needs to be constructed and used in the "+sip.instance" Contact
header field parameter for outbound behavior.
Jennings, et al. Standards Track [Page 13]

RFC 5626 Client-Initiated Connections in SIP October 2009
To convey its instance-id in both requests and responses, the UA
includes a "sip.instance" media feature tag as a UA characteristic
[RFC3840]. This media feature tag is encoded in the Contact header
field as the "+sip.instance" Contact header field parameter. One
case where a UA could prefer to omit the "sip.instance" media feature
tag is when it is making an anonymous request or some other privacy
concern requires that the UA not reveal its identity.
Note: [RFC3840] defines equality rules for callee capabilities
parameters, and according to that specification, the
"sip.instance" media feature tag will be compared by case-
sensitive string comparison. This means that the URN will be
encapsulated by angle brackets ("<" and ">") when it is placed
within the quoted string value of the "+sip.instance" Contact
header field parameter. The case-sensitive matching rules apply
only to the generic usages defined in the callee capabilities
[RFC3840] and the caller preferences [RFC3841] specifications.
When the instance ID is used in this specification, it is
"extracted" from the value in the "sip.instance" media feature
tag. Thus, equality comparisons are performed using the rules for
URN equality that are specific to the scheme in the URN. If the
element performing the comparisons does not understand the URN
scheme, it performs the comparisons using the lexical equality
rules defined in [RFC2141]. Lexical equality could result in two
URNs being considered unequal when they are actually equal. In
this specific usage of URNs, the only element that provides the
URN is the SIP UA instance identified by that URN. As a result,
the UA instance has to provide lexically equivalent URNs in each
registration it generates. This is likely to be normal behavior
in any case; clients are not likely to modify the value of the
instance ID so that it remains functionally equivalent to (yet
lexicographically different from) previous registrations.
4.2. Registrations4.2.1. Initial Registrations
At configuration time, UAs obtain one or more SIP URIs representing
the default outbound-proxy-set. This specification assumes the set
is determined via any of a number of configuration mechanisms, and
future specifications can define additional mechanisms such as using
DNS to discover this set. How the UA is configured is outside the
scope of this specification. However, a UA MUST support sets with at
least two outbound proxy URIs and SHOULD support sets with up to four
URIs.
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RFC 5626 Client-Initiated Connections in SIP October 2009
For each outbound proxy URI in the set, the User Agent Client (UAC)
SHOULD send a REGISTER request using this URI as the default outbound
proxy. (Alternatively, the UA could limit the number of flows formed
to conserve battery power, for example). If the set has more than
one URI, the UAC MUST send a REGISTER request to at least two of the
default outbound proxies from the set. UAs that support this
specification MUST include the outbound option tag in a Supported
header field in a REGISTER request. Each of these REGISTER requests
will use a unique Call-ID. Forming the route set for the request is
outside the scope of this document, but typically results in sending
the REGISTER such that the topmost Route header field contains a
loose route to the outbound proxy URI.
REGISTER requests, other than those described in Section 4.2.3, MUST
include an instance-id media feature tag as specified in Section 4.1.
A UAC conforming to this specification MUST include in the Contact
header field, a "reg-id" parameter that is distinct from other
"reg-id" parameters used in other registrations that use the same
"+sip.instance" Contact header field parameter and AOR. Each one of
these registrations will form a new flow from the UA to the proxy.
The sequence of reg-id values does not have to be sequential but MUST
be exactly the same sequence of reg-id values each time the UA
instance power cycles or reboots, so that the reg-id values will
collide with the previously used reg-id values. This is so the
registrar can replace the older registrations.
Note: The UAC can situationally decide whether to request outbound
behavior by including or omitting the "reg-id" Contact header
field parameter. For example, imagine the outbound-proxy-set
contains two proxies in different domains, EP1 and EP2. If an
outbound-style registration succeeded for a flow through EP1, the
UA might decide to include 'outbound' in its Require header field
when registering with EP2, in order to ensure consistency.
Similarly, if the registration through EP1 did not support
outbound, the UA might not register with EP2 at all.
The UAC MUST support the Path header [RFC3327] mechanism, and
indicate its support by including the 'path' option-tag in a
Supported header field value in its REGISTER requests. Other than
optionally examining the Path vector in the response, this is all
that is required of the UAC to support Path.
The UAC examines successful registration responses for the presence
of an outbound option-tag in a Require header field value. Presence
of this option-tag indicates that the registrar is compliant with
this specification, and that any edge proxies which needed to
participate are also compliant. If the registrar did not support
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RFC 5626 Client-Initiated Connections in SIP October 2009
outbound, the UA has potentially registered an un-routable contact.
It is the responsibility of the UA to remove any inappropriate
Contacts.
If outbound registration succeeded, as indicated by the presence of
the outbound option-tag in the Require header field of a successful
registration response, the UA begins sending keep-alives as described
in Section 4.4.
Note: The UA needs to honor 503 (Service Unavailable) responses to
registrations as described in [RFC3261] and [RFC3263]. In
particular, implementors should note that when receiving a 503
(Service Unavailable) response with a Retry-After header field,
the UA is expected to wait the indicated amount of time and retry
the registration. A Retry-After header field value of 0 is valid
and indicates the UA is expected to retry the REGISTER request
immediately. Implementations need to ensure that when retrying
the REGISTER request, they revisit the DNS resolution results such
that the UA can select an alternate host from the one chosen the
previous time the URI was resolved.
If the registering UA receives a 439 (First Hop Lacks Outbound
Support) response to a REGISTER request, it MAY re-attempt
registration without using the outbound mechanism (subject to local
policy at the client). If the client has one or more alternate
outbound proxies available, it MAY re-attempt registration through
such outbound proxies. See Section 11.6 for more information on the
439 response code.
4.2.2. Subsequent REGISTER Requests
Registrations for refreshing a binding and for removing a binding use
the same instance-id and reg-id values as the corresponding initial
registration where the binding was added. Registrations that merely
refresh an existing binding are sent over the same flow as the
original registration where the binding was added.
If a re-registration is rejected with a recoverable error response,
for example by a 503 (Service Unavailable) containing a Retry-After
header, the UAC SHOULD NOT tear down the corresponding flow if the
flow uses a connection-oriented transport such as TCP. As long as
"pongs" are received in response to "pings", the flow SHOULD be kept
active until a non-recoverable error response is received. This
prevents unnecessary closing and opening of connections.
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RFC 5626 Client-Initiated Connections in SIP October 20094.2.3. Third-Party Registrations
In an initial registration or re-registration, a UA MUST NOT include
a "reg-id" header field parameter in the Contact header field if the
registering UA is not the same instance as the UA referred to by the
target Contact header field. (This practice is occasionally used to
install forwarding policy into registrars.)
A UAC also MUST NOT include an instance-id feature tag or "reg-id"
Contact header field parameter in a request to un-register all
Contacts (a single Contact header field value with the value of "*").
4.3. Sending Non-REGISTER Requests
When a UAC is about to send a request, it first performs normal
processing to select the next hop URI. The UA can use a variety of
techniques to compute the route set and accordingly the next hop URI.
Discussion of these techniques is outside the scope of this document.
UAs that support this specification SHOULD include the outbound
option tag in a Supported header field in a request that is not a
REGISTER request.
The UAC performs normal DNS resolution on the next hop URI (as
described in [RFC3263]) to find a protocol, IP address, and port.
For protocols that don't use TLS, if the UAC has an existing flow to
this IP address, and port with the correct protocol, then the UAC
MUST use the existing connection. For TLS protocols, there MUST also
be a match between the host production in the next hop and one of the
URIs contained in the subjectAltName in the peer certificate. If the
UAC cannot use one of the existing flows, then it SHOULD form a new
flow by sending a datagram or opening a new connection to the next
hop, as appropriate for the transport protocol.
Typically, a UAC using the procedures of this document and sending a
dialog-forming request will want all subsequent requests in the
dialog to arrive over the same flow. If the UAC is using a Globally
Routable UA URI (GRUU) [RFC5627] that was instantiated using a
Contact header field value that included an "ob" parameter, the UAC
sends the request over the flow used for registration, and subsequent
requests will arrive over that same flow. If the UAC is not using
such a GRUU, then the UAC adds an "ob" parameter to its Contact
header field value. This will cause all subsequent requests in the
dialog to arrive over the flow instantiated by the dialog-forming
request. This case is typical when the request is sent prior to
registration, such as in the initial subscription dialog for the
configuration framework [CONFIG-FMWK].
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RFC 5626 Client-Initiated Connections in SIP October 2009
Note: If the UAC wants a UDP flow to work through NATs or
firewalls, it still needs to put the 'rport' parameter [RFC3581]
in its Via header field value, and send from the port it is
prepared to receive on. More general information about NAT
traversal in SIP is described in [NAT-SCEN].
4.4. Keep-Alives and Detecting Flow Failure
Keep-alives are used for refreshing NAT/firewall bindings and
detecting flow failure. Flows can fail for many reasons including
the rebooting of NATs and the crashing of edge proxies.
As described in Section 4.2, a UA that registers will begin sending
keep-alives after an appropriate registration response. A UA that
does not register (for example, a PSTN gateway behind a firewall) can
also send keep-alives under certain circumstances.
Under specific circumstances, a UAC might be allowed to send STUN
keep-alives even if the procedures in Section 4.2 were not completed,
provided that there is an explicit indication that the target first-
hop SIP node supports STUN keep-alives. For example, this applies to
a non-registering UA or to a case where the UA registration
succeeded, but the response did not include the outbound option-tag
in the Require header field.
Note: A UA can "always" send a double CRLF (a "ping") over
connection-oriented transports as this is already allowed by
Section 7.5 of [RFC3261]. However a UA that did not register
using outbound registration cannot expect a CRLF in response (a
"pong") unless the UA has an explicit indication that CRLF keep-
alives are supported as described in this section. Likewise, a UA
that did not successfully register with outbound procedures needs
explicit indication that the target first-hop SIP node supports
STUN keep-alives before it can send any STUN messages.
A configuration option indicating keep-alive support for a specific
target is considered an explicit indication. If these conditions are
satisfied, the UA sends its keep-alives according to the same
guidelines as those used when UAs register; these guidelines are
described below.
The UA needs to detect when a specific flow fails. The UA actively
tries to detect failure by periodically sending keep-alive messages
using one of the techniques described in Sections 4.4.1 or 4.4.2. If
a flow with a registration has failed, the UA follows the procedures
in Section 4.2 to form a new flow to replace the failed one.
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RFC 5626 Client-Initiated Connections in SIP October 2009
When a successful registration response contains the Flow-Timer
header field, the value of this header field is the number of seconds
the server is prepared to wait without seeing keep-alives before it
could consider the corresponding flow dead. Note that the server
would wait for an amount of time larger than the Flow-Timer in order
to have a grace period to account for transport delay. The UA MUST
send keep-alives at least as often as this number of seconds. If the
UA uses the server-recommended keep-alive frequency it SHOULD send
its keep-alives so that the interval between each keep-alive is
randomly distributed between 80% and 100% of the server-provided
time. For example, if the server suggests 120 seconds, the UA would
send each keep-alive with a different frequency between 95 and 120
seconds.
If no Flow-Timer header field was present in a register response for
this flow, the UA can send keep-alives at its discretion. The
sections below provide RECOMMENDED default values for these keep-
alives.
The client needs to perform normal [RFC3263] SIP DNS resolution on
the URI from the outbound-proxy-set to pick a transport. Once a
transport is selected, the UA selects the keep-alive approach that is
recommended for that transport.
Section 4.4.1 describes a keep-alive mechanism for connection-
oriented transports such as TCP or SCTP. Section 4.4.2 describes a
keep-alive mechanism for connection-less transports such as UDP.
Support for other transports such as DCCP [RFC4340] is for further
study.
4.4.1. Keep-Alive with CRLF
This approach MUST only be used with connection oriented transports
such as TCP or SCTP; it MUST NOT be used with connection-less
transports such as UDP.
A User Agent that forms flows checks if the configured URI to which
the UA is connecting resolves to a connection-oriented transport
(e.g., TCP and TLS over TCP).
For this mechanism, the client "ping" is a double-CRLF sequence, and
the server "pong" is a single CRLF, as defined in the ABNF below:
CRLF = CR LF
double-CRLF = CR LF CR LF
CR = %x0D
LF = %x0A
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RFC 5626 Client-Initiated Connections in SIP October 2009
The "ping" and "pong" need to be sent between SIP messages and cannot
be sent in the middle of a SIP message. If sending over TLS, the
CRLFs are sent inside the TLS protected channel. If sending over a
SigComp [RFC3320] compressed data stream, the CRLF keep-alives are
sent inside the compressed stream. The double CRLF is considered a
single SigComp message. The specific mechanism for representing
these characters is an implementation-specific matter to be handled
by the SigComp compressor at the sending end.
If a pong is not received within 10 seconds after sending a ping (or
immediately after processing any incoming message being received when
that 10 seconds expires), then the client MUST treat the flow as
failed. Clients MUST support this CRLF keep-alive.
Note: This value of 10-second timeout was selected to be long
enough that it allows plenty of time for a server to send a
response even if the server is temporarily busy with an
administrative activity. At the same time, it was selected to be
small enough that a UA registered to two redundant servers with
unremarkable hardware uptime could still easily provide very high
levels of overall reliability. Although some Internet protocols
are designed for round-trip times over 10 seconds, SIP for real-
time communications is not really usable in these type of
environments as users often abandon calls before waiting much more
than a few seconds.
When a Flow-Timer header field is not provided in the most recent
success registration response, the proper selection of keep-alive
frequency is primarily a trade-off between battery usage and
availability. The UA MUST select a random number between a fixed or
configurable upper bound and a lower bound, where the lower bound is
20% less then the upper bound. The fixed upper bound or the default
configurable upper bound SHOULD be 120 seconds (95 seconds for the
lower bound) where battery power is not a concern and 840 seconds
(672 seconds for the lower bound) where battery power is a concern.
The random number will be different for each keep-alive "ping".
Note on selection of time values: the 120-second upper bound was
chosen based on the idea that for a good user experience, failures
normally will be detected in this amount of time and a new
connection will be set up. The 14-minute upper bound for battery-
powered devices was selected based on NATs with TCP timeouts as
low as 15 minutes. Operators that wish to change the relationship
between load on servers and the expected time that a user might
not receive inbound communications will probably adjust this time.
The 95-second lower bound was chosen so that the jitter introduced
will result in a relatively even load on the servers after 30
minutes.
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RFC 5626 Client-Initiated Connections in SIP October 20094.4.2. Keep-Alive with STUN
This approach MUST only be used with connection-less transports, such
as UDP; it MUST NOT be used for connection-oriented transports such
as TCP and SCTP.
A User Agent that forms flows checks if the configured URI to which
the UA is connecting resolves to use the UDP transport. The UA can
periodically perform keep-alive checks by sending STUN [RFC5389]
Binding Requests over the flow as described in Section 8. Clients
MUST support STUN-based keep-alives.
When a Flow-Timer header field is not included in a successful
registration response, the time between each keep-alive request
SHOULD be a random number between 24 and 29 seconds.
Note on selection of time values: the upper bound of 29 seconds
was selected, as many NATs have UDP timeouts as low as 30 seconds.
The 24-second lower bound was selected so that after 10 minutes
the jitter introduced by different timers will make the keep-alive
requests unsynchronized to evenly spread the load on the servers.
Note that the short NAT timeouts with UDP have a negative impact
on battery life.
If a STUN Binding Error Response is received, or if no Binding
Response is received after 7 retransmissions (16 times the STUN "RTO"
timer -- where RTO is an estimate of round-trip time), the UA
considers the flow failed. If the XOR-MAPPED-ADDRESS in the STUN
Binding Response changes, the UA MUST treat this event as a failure
on the flow.
4.5. Flow Recovery
When a flow used for registration (through a particular URI in the
outbound-proxy-set) fails, the UA needs to form a new flow to replace
the old flow and replace any registrations that were previously sent
over this flow. Each new registration MUST have the same reg-id
value as the registration it replaces. This is done in much the same
way as forming a brand new flow as described in Section 4.2; however,
if there is a failure in forming this flow, the UA needs to wait a
certain amount of time before retrying to form a flow to this
particular next hop.
The amount of time to wait depends if the previous attempt at
establishing a flow was successful. For the purposes of this
section, a flow is considered successful if outbound registration
succeeded, and if keep-alives are in use on this flow, at least one
subsequent keep-alive response was received.
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RFC 5626 Client-Initiated Connections in SIP October 2009
The number of seconds to wait is computed in the following way. If
all of the flows to every URI in the outbound proxy set have failed,
the base-time is set to a lower value (with a default of 30 seconds);
otherwise, in the case where at least one of the flows has not
failed, the base-time is set to a higher value (with a default of 90
seconds). The upper-bound wait time (W) is computed by taking two
raised to the power of the number of consecutive registration
failures for that URI, and multiplying this by the base-time, up to a
configurable maximum time (with a default of 1800 seconds).
W = min (max-time, (base-time * (2 ^ consecutive-failures)))
These times MAY be configurable in the UA. The three times are:
o max-time with a default of 1800 seconds
o base-time (if all failed) with a default of 30 seconds
o base-time (if all have not failed) with a default of 90 seconds
For example, if the base-time is 30 seconds, and there were three
failures, then the upper-bound wait time is min(1800, 30*(2^3)) or
240 seconds. The actual amount of time the UA waits before retrying
registration (the retry delay time) is computed by selecting a
uniform random time between 50 and 100% of the upper-bound wait time.
The UA MUST wait for at least the value of the retry delay time
before trying another registration to form a new flow for that URI (a
503 response to an earlier failed registration attempt with a Retry-
After header field value may cause the UA to wait longer).
To be explicitly clear on the boundary conditions: when the UA boots,
it immediately tries to register. If this fails and no registration
on other flows succeed, the first retry happens somewhere between 30
and 60 seconds after the failure of the first registration request.
If the number of consecutive-failures is large enough that the
maximum of 1800 seconds is reached, the UA will keep trying
indefinitely with a random time of 15 to 30 minutes between each
attempt.
5. Edge Proxy Procedures5.1. Processing Register Requests
When an edge proxy receives a registration request with a "reg-id"
header field parameter in the Contact header field, it needs to
determine if it (the edge proxy) will have to be visited for any
subsequent requests sent to the User Agent identified in the Contact
header field, or not. If the edge proxy is the first hop, as
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RFC 5626 Client-Initiated Connections in SIP October 2009
indicated by the Via header field, it MUST insert its URI in a Path
header field value as described in [RFC3327]. If it is not the first
hop, it might still decide to add itself to the Path header based on
local policy. In addition, if the edge proxy is the first SIP node
after the UAC, the edge proxy either MUST store a "flow token"
(containing information about the flow from the previous hop) in its
Path URI or reject the request. The flow token MUST be an identifier
that is unique to this network flow. The flow token MAY be placed in
the userpart of the URI. In addition, the first node MUST include an
"ob" URI parameter in its Path header field value. If the edge proxy
is not the first SIP node after the UAC it MUST NOT place an "ob" URI
parameter in a Path header field value. The edge proxy can determine
if it is the first hop by examining the Via header field.
5.2. Generating Flow Tokens
A trivial but impractical way to satisfy the flow token requirement
in Section 5.1 involves storing a mapping between an incrementing
counter and the connection information; however, this would require
the edge proxy to keep an infeasible amount of state. It is unclear
when this state could be removed, and the approach would have
problems if the proxy crashed and lost the value of the counter. A
stateless example is provided below. A proxy can use any algorithm
it wants as long as the flow token is unique to a flow, the flow can
be recovered from the token, and the token cannot be modified by
attackers.
Example Algorithm: When the proxy boots, it selects a 20-octet
crypto random key called K that only the edge proxy knows. A byte
array, called S, is formed that contains the following information
about the flow the request was received on: an enumeration
indicating the protocol, the local IP address and port, the remote
IP address and port. The HMAC of S is computed using the key K
and the HMAC-SHA1-80 algorithm, as defined in [RFC2104]. The
concatenation of the HMAC and S are base64 encoded, as defined in
[RFC4648], and used as the flow identifier. When using IPv4
addresses, this will result in a 32-octet identifier.
5.3. Forwarding Non-REGISTER Requests
When an edge proxy receives a request, it applies normal routing
procedures with the following additions. If the edge proxy receives
a request where the edge proxy is the host in the topmost Route
header field value, and the Route header field value contains a flow
token, the proxy follows the procedures of this section. Otherwise
the edge proxy skips the procedures in this section, removes itself
from the Route header field, and continues processing the request.
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RFC 5626 Client-Initiated Connections in SIP October 2009
The proxy decodes the flow token and compares the flow in the flow
token with the source of the request to determine if this is an
"incoming" or "outgoing" request.
If the flow in the flow token identified by the topmost Route header
field value matches the source IP address and port of the request,
the request is an "outgoing" request; otherwise, it is an "incoming"
request.
5.3.1. Processing Incoming Requests
If the Route header value contains an "ob" URI parameter, the Route
header was probably copied from the Path header in a registration.
If the Route header value contains an "ob" URI parameter, and the
request is a new dialog-forming request, the proxy needs to adjust
the route set to ensure that subsequent requests in the dialog can be
delivered over a valid flow to the UA instance identified by the flow
token.
Note: A simple approach to satisfy this requirement is for the
proxy to add a Record-Route header field value that contains the
flow-token, by copying the URI in the Route header minus the "ob"
parameter.
Next, whether the Route header field contained an "ob" URI parameter
or not, the proxy removes the Route header field value and forwards
the request over the 'logical flow' identified by the flow token,
that is known to deliver data to the specific target UA instance. If
the flow token has been tampered with, the proxy SHOULD send a 403
(Forbidden) response. If the flow no longer exists, the proxy SHOULD
send a 430 (Flow Failed) response to the request.
Proxies that used the example algorithm described in Section 5.2 to
form a flow token follow the procedures below to determine the
correct flow. To decode the flow token, take the flow identifier in
the user portion of the URI and base64 decode it, then verify the
HMAC is correct by recomputing the HMAC and checking that it matches.
If the HMAC is not correct, the request has been tampered with.
5.3.2. Processing Outgoing Requests
For mid-dialog requests to work with outbound UAs, the requests need
to be forwarded over some valid flow to the appropriate UA instance.
If the edge proxy receives an outgoing dialog-forming request, the
edge proxy can use the presence of the "ob" URI parameter in the
UAC's Contact URI (or topmost Route header field) to determine if the
edge proxy needs to assist in mid-dialog request routing.
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RFC 5626 Client-Initiated Connections in SIP October 2009
Implementation note: Specific procedures at the edge proxy to
ensure that mid-dialog requests are routed over an existing flow
are not part of this specification. However, an approach such as
having the edge proxy add a Record-Route header with a flow token
is one way to ensure that mid-dialog requests are routed over the
correct flow.
5.4. Edge Proxy Keep-Alive Handling
All edge proxies compliant with this specification MUST implement
support for STUN NAT keep-alives on their SIP UDP ports as described
in Section 8.
When a server receives a double CRLF sequence between SIP messages on
a connection-oriented transport such as TCP or SCTP, it MUST
immediately respond with a single CRLF over the same connection.
The last proxy to forward a successful registration response to a UA
MAY include a Flow-Timer header field if the response contains the
outbound option-tag in a Require header field value in the response.
The reason a proxy would send a Flow-Timer is if it wishes to detect
flow failures proactively and take appropriate action (e.g., log
alarms, provide alternative treatment if incoming requests for the UA
are received, etc.). The server MUST wait for an amount of time
larger than the Flow-Timer in order to have a grace period to account
for transport delay.
6. Registrar Procedures
This specification updates the definition of a binding in [RFC3261],
Section 10 and [RFC3327], Section 5.3.
Registrars that implement this specification MUST support the Path
header mechanism [RFC3327].
When receiving a REGISTER request, the registrar MUST check from its
Via header field if the registrar is the first hop or not. If the
registrar is not the first hop, it MUST examine the Path header of
the request. If the Path header field is missing or it exists but
the first URI does not have an "ob" URI parameter, then outbound
processing MUST NOT be applied to the registration. In this case,
the following processing applies: if the REGISTER request contains
the reg-id and the outbound option tag in a Supported header field,
then the registrar MUST respond to the REGISTER request with a 439
(First Hop Lacks Outbound Support) response; otherwise, the registrar
MUST ignore the "reg-id" parameter of the Contact header. See
Section 11.6 for more information on the 439 response code.
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RFC 5626 Client-Initiated Connections in SIP October 2009
A Contact header field value with an instance-id media feature tag
but no "reg-id" header field parameter is valid (this combination
will result in the creation of a GRUU, as described in the GRUU
specification [RFC5627]), but one with a reg-id but no instance-id is
not valid. If the registrar processes a Contact header field value
with a reg-id but no instance-id, it simply ignores the reg-id
parameter.
A registration containing a "reg-id" header field parameter and a
non-zero expiration is used to register a single UA instance over a
single flow, and can also de-register any Contact header fields with
zero expiration. Therefore, if the Contact header field contains
more than one header field value with a non-zero expiration and any
of these header field values contain a "reg-id" Contact header field
parameter, the entire registration SHOULD be rejected with a 400 (Bad
Request) response. The justification for recommending rejection
versus making it mandatory is that the receiver is allowed by
[RFC3261] to squelch (not respond to) excessively malformed or
malicious messages.
If the Contact header did not contain a "reg-id" Contact header field
parameter or if that parameter was ignored (as described above), the
registrar MUST NOT include the outbound option-tag in the Require
header field of its response.
The registrar MUST be prepared to receive, simultaneously for the
same AOR, some registrations that use instance-id and reg-id and some
registrations that do not. The registrar MAY be configured with
local policy to reject any registrations that do not include the
instance-id and reg-id, or with Path header field values that do not
contain the "ob" URI parameter. If the Contact header field does not
contain a "+sip.instance" Contact header field parameter, the
registrar processes the request using the Contact binding rules in
[RFC3261].
When a "+sip.instance" Contact header field parameter and a "reg-id"
Contact header field parameter are present in a Contact header field
of a REGISTER request (after the Contact header validation as
described above), the corresponding binding is between an AOR and the
combination of the instance-id (from the "+sip.instance" Contact
header parameter) and the value of "reg-id" Contact header field
parameter parameter. The registrar MUST store in the binding the
Contact URI, all the Contact header field parameters, and any Path
header field values. (Even though the Contact URI is not used for
binding comparisons, it is still needed by the authoritative proxy to
form the target set.) Provided that the UAC had included an outbound
option-tag (defined in Section 11.4) in a Supported header field
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RFC 5626 Client-Initiated Connections in SIP October 2009
value in the REGISTER request, the registrar MUST include the
outbound option-tag in a Require header field value in its response
to that REGISTER request.
If the UAC has a direct flow with the registrar, the registrar MUST
store enough information to uniquely identify the network flow over
which the request arrived. For common operating systems with TCP,
this would typically be just the handle to the file descriptor where
the handle would become invalid if the TCP session was closed. For
common operating systems with UDP this would typically be the file
descriptor for the local socket that received the request, the local
interface, and the IP address and port number of the remote side that
sent the request. The registrar MAY store this information by adding
itself to the Path header field with an appropriate flow token.
If the registrar receives a re-registration for a specific
combination of AOR, and instance-id and reg-id values, the registrar
MUST update any information that uniquely identifies the network flow
over which the request arrived if that information has changed, and
SHOULD update the time the binding was last updated.
To be compliant with this specification, registrars that can receive
SIP requests directly from a UAC without intervening edge proxies
MUST implement the same keep-alive mechanisms as edge proxies
(Section 5.4). Registrars with a direct flow with a UA MAY include a
Flow-Timer header in a 2xx class registration response that includes
the outbound option-tag in the Require header.
7. Authoritative Proxy Procedures: Forwarding Requests
When a proxy uses the location service to look up a registration
binding and then proxies a request to a particular contact, it
selects a contact to use normally, with a few additional rules:
o The proxy MUST NOT populate the target set with more than one
contact with the same AOR and instance-id at a time.
o If a request for a particular AOR and instance-id fails with a 430
(Flow Failed) response, the proxy SHOULD replace the failed branch
with another target (if one is available) with the same AOR and
instance-id, but a different reg-id.
o If the proxy receives a final response from a branch other than a
408 (Request Timeout) or a 430 (Flow Failed) response, the proxy
MUST NOT forward the same request to another target representing
the same AOR and instance-id. The targeted instance has already
provided its response.
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RFC 5626 Client-Initiated Connections in SIP October 2009
The proxy uses the next-hop target of the message and the value of
any stored Path header field vector in the registration binding to
decide how to forward and populate the Route header in the request.
If the proxy is co-located with the registrar and stored information
about the flow to the UA that created the binding, then the proxy
MUST send the request over the same 'logical flow' saved with the
binding, since that flow is known to deliver data to the specific
target UA instance's network flow that was saved with the binding.
Implementation note: Typically this means that for TCP, the
request is sent on the same TCP socket that received the REGISTER
request. For UDP, the request is sent from the same local IP
address and port over which the registration was received, to the
same IP address and port from which the REGISTER was received.
If a proxy or registrar receives information from the network that
indicates that no future messages will be delivered on a specific
flow, then the proxy MUST invalidate all the bindings in the target
set that use that flow (regardless of AOR). Examples of this are a
TCP socket closing or receiving a destination unreachable ICMP error
on a UDP flow. Similarly, if a proxy closes a file descriptor, it
MUST invalidate all the bindings in the target set with flows that
use that file descriptor.
8. STUN Keep-Alive Processing
This section describes changes to the SIP transport layer that allow
SIP and STUN [RFC5389] Binding Requests to be mixed over the same
flow. This constitutes a new STUN usage. The STUN messages are used
to verify that connectivity is still available over a UDP flow, and
to provide periodic keep-alives. These STUN keep-alives are always
sent to the next SIP hop. STUN messages are not delivered end-to-
end.
The only STUN messages required by this usage are Binding Requests,
Binding Responses, and Binding Error Responses. The UAC sends
Binding Requests over the same UDP flow that is used for sending SIP
messages. These Binding Requests do not require any STUN attributes.
The corresponding Binding Responses do not require any STUN
attributes except the XOR-MAPPED-ADDRESS. The UAS, proxy, or
registrar responds to a valid Binding Request with a Binding Response
that MUST include the XOR-MAPPED-ADDRESS attribute.
If a server compliant to this section receives SIP requests on a
given interface and UDP port, it MUST also provide a limited version
of a STUN server on the same interface and UDP port.
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RFC 5626 Client-Initiated Connections in SIP October 2009
Note: It is easy to distinguish STUN and SIP packets sent over
UDP, because the first octet of a STUN Binding method has a value
of 0 or 1, while the first octet of a SIP message is never a 0 or
1.
Because sending and receiving binary STUN data on the same ports used
for SIP is a significant and non-backwards compatible change to RFC3261, this section requires a number of checks before sending STUN
messages to a SIP node. If a SIP node sends STUN requests (for
example, due to incorrect configuration) despite these warnings, the
node could be blacklisted for UDP traffic.
A SIP node MUST NOT send STUN requests over a flow unless it has an
explicit indication that the target next-hop SIP server claims to
support this specification. UACs MUST NOT use an ambiguous
configuration option such as "Work through NATs?" or "Do keep-
alives?" to imply next-hop STUN support. A UAC MAY use the presence
of an "ob" URI parameter in the Path header in a registration
response as an indication that its first edge proxy supports the
keep-alives defined in this document.
Note: Typically, a SIP node first sends a SIP request and waits to
receive a 2xx class response over a flow to a new target
destination, before sending any STUN messages. When scheduled for
the next NAT refresh, the SIP node sends a STUN request to the
target.
Once a flow is established, failure of a STUN request (including its
retransmissions) is considered a failure of the underlying flow. For
SIP over UDP flows, if the XOR-MAPPED-ADDRESS returned over the flow
changes, this indicates that the underlying connectivity has changed,
and is considered a flow failure.
The SIP keep-alive STUN usage requires no backwards compatibility
with [RFC3489].
8.1. Use with SigComp
When STUN is used together with SigComp [RFC3320] compressed SIP
messages over the same flow, the STUN messages are simply sent
uncompressed, "outside" of SigComp. This is supported by
multiplexing STUN messages with SigComp messages by checking the two
topmost bits of the message. These bits are always one for SigComp,
or zero for STUN.
Note: All SigComp messages contain a prefix (the five most
significant bits of the first byte are set to one) that does not
occur in UTF-8 [RFC3629] encoded text messages, so for
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RFC 5626 Client-Initiated Connections in SIP October 2009
applications that use this encoding (or ASCII encoding) it is
possible to multiplex uncompressed application messages and
SigComp messages on the same UDP port. The most significant two
bits of every STUN Binding method are both zeroes. This, combined
with the magic cookie, aids in differentiating STUN packets from
other protocols when STUN is multiplexed with other protocols on
the same port.
9. Example Message Flow
Below is an example message flow illustrating most of the concepts
discussed in this specification. In many cases, Via, Content-Length,
and Max-Forwards headers are omitted for brevity and readability.
In these examples, "EP1" and "EP2" are outbound proxies, and "Proxy"
is the authoritativeProxy.
The section is subdivided into independent calls flows; however, they
are structured in sequential order of a hypothetical sequence of call
flows.
9.1. Subscription to Configuration Package
If the outbound proxy set is already configured on Bob's UA, then
this subsection can be skipped. Otherwise, if the outbound proxy set
is learned through the configuration package, Bob's UA sends a
SUBSCRIBE request for the UA profile configuration package
[CONFIG-FMWK]. This request is a poll (Expires is zero). After
receiving the NOTIFY request, Bob's UA fetches the external
configuration using HTTPS (not shown) and obtains a configuration
file that contains the outbound-proxy-set "sip:ep1.example.com;lr"
and "sip:ep2.example.com;lr".
[----example.com domain-------------------------]
Bob EP1 EP2 Proxy Config
| | | | |
1)|SUBSCRIBE->| | | |
2)| |---SUBSCRIBE Event: ua-profile ->|
3)| |<--200 OK -----------------------|
4)|<--200 OK--| | | |
5)| |<--NOTIFY------------------------|
6)|<--NOTIFY--| | | |
7)|---200 OK->| | | |
8)| |---200 OK ---------------------->|
| | | | |
In this example, the DNS server happens to be configured so that sip:
example.com resolves to EP1 and EP2.
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RFC 5626 Client-Initiated Connections in SIP October 200910. Grammar
This specification defines a new header field "Flow-Timer", and new
Contact header field parameters, "reg-id" and "+sip.instance". The
grammar includes the definitions from [RFC3261]. Flow-Timer is an
extension-header from the message-header in the [RFC3261] ABNF.
The ABNF [RFC5234] is:
Flow-Timer = "Flow-Timer" HCOLON 1*DIGIT
contact-params =/ c-p-reg / c-p-instance
c-p-reg = "reg-id" EQUAL 1*DIGIT ; 1 to (2^31 - 1)
c-p-instance = "+sip.instance" EQUAL
DQUOTE "<" instance-val ">" DQUOTE
instance-val = 1*uric ; defined in RFC 3261
The value of the reg-id MUST NOT be 0 and MUST be less than 2^31.
11. IANA Considerations11.1. Flow-Timer Header Field
This specification defines a new SIP header field "Flow-Timer" whose
syntax is defined in Section 10.
Header Name compact Reference
----------------- ------- ---------
Flow-Timer [RFC5626]
11.2. "reg-id" Contact Header Field Parameter
This specification defines a new Contact header field parameter
called reg-id in the "Header Field Parameters and Parameter Values"
sub-registry as per the registry created by [RFC3968]. The syntax is
defined in Section 10. The required information is:
Predefined
Header Field Parameter Name Values Reference
---------------------- --------------------- ---------- ---------
Contact reg-id No [RFC5626]
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RFC 5626 Client-Initiated Connections in SIP October 200911.3. SIP/SIPS URI Parameters
This specification augments the "SIP/SIPS URI Parameters" sub-
registry as per the registry created by [RFC3969]. The required
information is:
Parameter Name Predefined Values Reference
-------------- ----------------- ---------
ob No [RFC5626]
11.4. SIP Option Tag
This specification registers a new SIP option tag, as per the
guidelines in Section 27.1 of [RFC3261].
Name: outbound
Description: This option-tag is used to identify UAs and registrars
that support extensions for Client-Initiated Connections. A UA
places this option in a Supported header to communicate its
support for this extension. A registrar places this option-tag in
a Require header to indicate to the registering User Agent that
the registrar used registrations using the binding rules defined
in this extension.
11.5. 430 (Flow Failed) Response Code
This document registers a new SIP response code (430 Flow Failed), as
per the guidelines in Section 27.4 of [RFC3261]. This response code
is used by an edge proxy to indicate to the Authoritative Proxy that
a specific flow to a UA instance has failed. Other flows to the same
instance could still succeed. The Authoritative Proxy SHOULD attempt
to forward to another target (flow) with the same instance-id and
AOR. Endpoints should never receive a 430 response. If an endpoint
receives a 430 response, it should treat it as a 400 (Bad Request)
per normal procedures, as in Section 8.1.3.2 of [RFC3261]. This
response code is defined by the following information, which has been
added to the method and response-code sub-registry under the SIP
Parameters registry.
Response Code Reference
------------------------------------------ ---------
Request Failure 4xx
430 Flow Failed [RFC5626]
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RFC 5626 Client-Initiated Connections in SIP October 200911.6. 439 (First Hop Lacks Outbound Support) Response Code
This document registers a new SIP response code (439 First Hop Lacks
Outbound Support), as per the guidelines in Section 27.4 of
[RFC3261]. This response code is used by a registrar to indicate
that it supports the 'outbound' feature described in this
specification, but that the first outbound proxy that the user is
attempting to register through does not. Note that this response
code is only appropriate in the case that the registering User Agent
advertises support for outbound processing by including the outbound
option tag in a Supported header field. Proxies MUST NOT send a 439
response to any requests that do not contain a "reg-id" parameter and
an outbound option tag in a Supported header field. This response
code is defined by the following information, which has been added to
the method and response-code sub-registry under the SIP Parameters
registry.
Response Code Reference
------------------------------------------ ---------
Request Failure 4xx
439 First Hop Lacks Outbound Support [RFC&rfc.number;]
11.7. Media Feature Tag
This section registers a new media feature tag, per the procedures
defined in [RFC2506]. The tag is placed into the sip tree, which is
defined in [RFC3840].
Media feature tag name: sip.instance
ASN.1 Identifier: 23
Summary of the media feature indicated by this tag: This feature tag
contains a string containing a URN that indicates a unique
identifier associated with the UA instance registering the
Contact.
Values appropriate for use with this feature tag: String (equality
relationship).
The feature tag is intended primarily for use in the following
applications, protocols, services, or negotiation mechanisms:
This feature tag is most useful in a communications application,
for describing the capabilities of a device, such as a phone or
PDA.
Examples of typical use: Routing a call to a specific device.
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RFC 5626 Client-Initiated Connections in SIP October 2009
Related standards or documents: RFC 5626
Security Considerations: This media feature tag can be used in ways
which affect application behaviors. For example, the SIP caller
preferences extension [RFC3841] allows for call routing decisions
to be based on the values of these parameters. Therefore, if an
attacker can modify the values of this tag, they might be able to
affect the behavior of applications. As a result, applications
that utilize this media feature tag SHOULD provide a means for
ensuring its integrity. Similarly, this feature tag should only
be trusted as valid when it comes from the user or User Agent
described by the tag. As a result, protocols for conveying this
feature tag SHOULD provide a mechanism for guaranteeing
authenticity.
12. Security Considerations
One of the key security concerns in this work is making sure that an
attacker cannot hijack the sessions of a valid user and cause all
calls destined to that user to be sent to the attacker. Note that
the intent is not to prevent existing active attacks on SIP UDP and
TCP traffic, but to ensure that no new attacks are added by
introducing the outbound mechanism.
The simple case is when there are no edge proxies. In this case, the
only time an entry can be added to the routing for a given AOR is
when the registration succeeds. SIP already protects against
attackers being able to successfully register, and this scheme relies
on that security. Some implementers have considered the idea of just
saving the instance-id without relating it to the AOR with which it
registered. This idea will not work because an attacker's UA can
impersonate a valid user's instance-id and hijack that user's calls.
The more complex case involves one or more edge proxies. When a UA
sends a REGISTER request through an edge proxy on to the registrar,
the edge proxy inserts a Path header field value. If the
registration is successfully authenticated, the registrar stores the
value of the Path header field. Later, when the registrar forwards a
request destined for the UA, it copies the stored value of the Path
header field into the Route header field of the request and forwards
the request to the edge proxy.
The only time an edge proxy will route over a particular flow is when
it has received a Route header that has the flow identifier
information that it has created. An incoming request would have
gotten this information from the registrar. The registrar will only
save this information for a given AOR if the registration for the AOR
has been successful; and the registration will only be successful if
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RFC 5626 Client-Initiated Connections in SIP October 2009
the UA can correctly authenticate. Even if an attacker has spoofed
some bad information in the Path header sent to the registrar, the
attacker will not be able to get the registrar to accept this
information for an AOR that does not belong to the attacker. The
registrar will not hand out this bad information to others, and
others will not be misled into contacting the attacker.
The Security Considerations discussed in [RFC3261] and [RFC3327] are
also relevant to this document. For the security considerations of
generating flow tokens, please also see Section 5.2. A discussion of
preventing the avalanche restart problem is in Section 4.5.
This document does not change the mandatory-to-implement security
mechanisms in SIP. User Agents are already required to implement
Digest authentication while support of TLS is recommended; proxy
servers are already required to implement Digest and TLS.
13. Operational Notes on Transports
This entire section is non-normative.
[RFC3261] requires proxies, registrars, and User Agents to implement
both TCP and UDP but deployments can chose which transport protocols
they want to use. Deployments need to be careful in choosing what
transports to use. Many SIP features and extensions, such as large
presence notification bodies, result in SIP requests that can be too
large to be reasonably transported over UDP. [RFC3261] states that
when a request is too large for UDP, the device sending the request
attempts to switch over to TCP. It is important to note that when
using outbound, this will only work if the UA has formed both UDP and
TCP outbound flows. This specification allows the UA to do so, but
in most cases it will probably make more sense for the UA to form a
TCP outbound connection only, rather than forming both UDP and TCP
flows. One of the key reasons that many deployments choose not to
use TCP has to do with the difficulty of building proxies that can
maintain a very large number of active TCP connections. Many
deployments today use SIP in such a way that the messages are small
enough that they work over UDP but they can not take advantage of all
the functionality SIP offers. Deployments that use only UDP outbound
connections are going to fail with sufficiently large SIP messages.
14. Requirements
This specification was developed to meet the following requirements:
1. Must be able to detect that a UA supports these mechanisms.
2. Support UAs behind NATs.
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RFC 5626 Client-Initiated Connections in SIP October 2009
3. Support TLS to a UA without a stable DNS name or IP address.
4. Detect failure of a connection and be able to correct for this.
5. Support many UAs simultaneously rebooting.
6. Support a NAT rebooting or resetting.
7. Minimize initial startup load on a proxy.
8. Support architectures with edge proxies.
15. Acknowledgments
Francois Audet acted as document shepherd for this document, tracking
hundreds of comments and incorporating many grammatical fixes as well
as prodding the editors to "get on with it". Jonathan Rosenberg,
Erkki Koivusalo, and Byron Campen provided many comments and useful
text. Dave Oran came up with the idea of using the most recent
registration first in the proxy. Alan Hawrylyshen co-authored the
document that formed the initial text of this specification.
Additionally, many of the concepts here originated at a connection
reuse meeting at IETF 60 that included the authors, Jon Peterson,
Jonathan Rosenberg, Alan Hawrylyshen, and Paul Kyzivat. The TCP
design team consisting of Chris Boulton, Scott Lawrence, Rajnish
Jain, Vijay K. Gurbani, and Ganesh Jayadevan provided input and text.
Nils Ohlmeier provided many fixes and initial implementation
experience. In addition, thanks to the following folks for useful
comments: Francois Audet, Flemming Andreasen, Mike Hammer, Dan Wing,
Srivatsa Srinivasan, Dale Worely, Juha Heinanen, Eric Rescorla,
Lyndsay Campbell, Christer Holmberg, Kevin Johns, Jeroen van Bemmel,
Derek MacDonald, Dean Willis, and Robert Sparks.
16. References16.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC2141] Moats, R., "URN Syntax", RFC 2141, May 1997.
[RFC2506] Holtman, K., Mutz, A., and T. Hardie, "Media Feature
Tag Registration Procedure", BCP 31, RFC 2506,
March 1999.
Jennings, et al. Standards Track [Page 45]